Re: Iron man and Hulk
They connected on an intellectual level, plus Stark got a kick out of a meek little nerdy dude unleashing something so big and destructive as Hulk. He seemed fascinated by the whole concept, believing that deep down a part of Hulk and his rage came from Banner.
And Banner appreciated someone treating him like a normal scientist, as well as addressing his Hulk alter ego with straightforward wry humor, as opposed to tiptoeing around the subject in fear as most tend to do.
There's more to it of course, but I'd say that's the gist of their bromance.